Device for controlling an electronically-monitored ultrasonic piezoelectric actuator, and method for using the same

ABSTRACT

The invention concerns a device for controlling an ultrasonic piezoelectric actuator, electronically monitored from a computer controlling a continuous current voltage source. The invention is characterized in that it comprises a DC to DC converter, powered by the voltage source, delivering at least in output a direct current voltage (Vs) between two end terminals (B 1,  B 2 ) parallel to which is connected at least an arm, consisting of two alternately controllable series-connected bridge switches and whereof the midpoint is alternately connected to the two output terminals of the DC to DC converter by a load consisting of at least an actuator in series with a resonance inductor.

The present invention relates to a device for operating electronicallydriven ultrasonic piezoelectric actuators, and more particularly fuelinjectors that have a piezoelectric stage and that are driven by theelectronic injection computer of an internal combustion engine in amotor vehicle. It also relates to a method for use of the said device.

More precisely, the problem that the invention is intended to solve isthe excitation of piezoelectric cells to cause vibration of thestructure of an injector described in French Patent Application No.99-14548, filed in the name of the Applicant. This type of injector isintended to atomize the fuel very finely into droplets that have thespecific size to assure precise dosing and that are sufficiently smallto assure complete and homogeneous vaporization of the injected fuel.Such an injector is provided with, among other parts, a cylindricalnozzle fed with fuel and having at its end an injection orifice, andwith means for causing cyclic vibration of the nozzle, such as atransducer provided with a piezoelectric ceramic stage, at the terminalsof which the electric voltage is varied within a scaling ratio in orderto modify its thickness between two extreme positions corresponding toopening and closing of the injector. An injector piezoelectric ceramicis equivalent on the first order to a capacitance having high chargingvoltage, in excess of one hundred volts. This transducer is driventime-wise and intensity-wise by the engine's electronic control systemin order to achieve oscillating opening of the nozzle tip at ultrasonicfrequency.

The purpose of the present invention is to generate a high-frequencyalternating signal to excite the piezoelectric cells from adirect-current voltage source. In a motor vehicle, the battery oralternator delivers a supply voltage having a value of 12 or 42 volts,which entails stepping up this voltage with a DC-to-DC voltage stepupconverter.

To this end, a first object of the invention is a device for operatingat least one ultrasonic piezoelectric actuator, which is drivenelectronically from a control computer and a direct-current voltagesource, characterized in that it is provided with a DC-to-DC converterthat is fed by the voltage source and that delivers at least onedirect-current output voltage between two end terminals, in parallelwith which there is connected at least one bridge arm composed of twoalternately operable switches in series, the midpoint of the arm beingconnected alternately to the two output terminals of the DC-to-DCconverter via a load composed of at least one actuator in series with aresonance inductor.

According to another characteristic of the invention, the DC-to-DCconverter delivers a single direct-current output voltage between thetwo end terminals, and at least one first bridge arm of the operatingdevice, composed of two alternately operable bridge switches in series,is connected in parallel across the output voltage, the midpoint of thesaid arm being connected via a load composed of at least one actuator inseries with a resonance inductor to the midpoint of at least one secondbridge arm, composed of two alternately operable bridge switches andmounted in parallel with the first arm.

According to another characteristic of the invention, wherein theDC-to-DC converter delivers a single direct-current output voltage, theoperating device is provided with a first bridge arm, composed of twoalternately operable switches and connected in parallel with the outputterminals of the converter, the midpoint of the said arm being connectedto the midpoint of a second bridge arm, composed of two alternatelyoperable switches and mounted in parallel with the first arm, via a loadcomprising four actuators in parallel, connected alternately to aresonance inductor.

According to another characteristic of the invention, wherein theDC-to-DC converter delivers a single direct-current output voltage, theoperating device is provided with at least one first bridge arm,composed of two alternately operable switches, and with at least twosecond bridge arms, each composed of two alternately operable switchesand mounted in parallel with the first arm between the two end terminalsof the converter, such that the midpoint of each first arm is connectedto the midpoint of at least one second arm via a load comprising atleast one actuator connected to a resonance inductor.

According to another characteristic of the invention, the DC-to-DCconverter delivers two direct-current output voltages between a commonreference terminal and two end terminals, in parallel with which thereis connected at least one bridge arm, composed of two alternatelyoperable switches, the midpoint of the said arm being connected to thereference terminal via a load comprising at least one actuator in serieswith a resonance inductor.

According to another characteristic of the invention, wherein theDC-to-DC converter delivers two direct-current output voltages, it isprovided with four first bridge arms, each composed of two alternatelyoperable bridge switches (P_(1i), P_(2i)) mounted in parallel betweenthe two end terminals (B₁, B₂), the midpoint (J_(i)) of the said armsbeing connected to the reference terminal (B_(o)) via a load composed ofan actuator (I_(i)) in series with a resonance inductor (L_(i)).

A second object of the invention is a method for use of a device foroperating at least one ultrasonic piezoelectric actuator, composed of abridge assembly, characterized in that, for operation of a givenactuator, the control computer on the one hand causes selection meansconnected to the said actuator to close and on the other hand, in afirst phase, causes a first pair of bridge switches composed of a firstswitch of a first bridge arm and of a second switch of a second bridgearm to close and simultaneously the second pair formed from the othertwo switches of the said arms to open and, in a second phase, the saidfour switches to change over to an inverse position, in such a way as toobtain a sinusoidal voltage at the terminals of the oscillating circuitformed by the said actuator and the associated resonance inductor, thesetwo phases being repeated a specified number of times during the periodof functioning of the actuator in order to generate a high-voltage,high-frequency signal at the piezoelectric actuator from thedirect-current voltage source.

According to another characteristic of the invention, for operation of agiven actuator, the control computer on the one hand causes selectionmeans connected to the said actuator to close and on the other hand, ina first phase, a first switch of a first bridge arm to close andsimultaneously the second bridge switch to open and, in a second phase,the said two switches to change over to an inverse position, in such away as to obtain a sinusoidal voltage at the terminals of theoscillating circuit formed by the said actuator and the associatedresonance inductor, these two phases being repeated a specified numberof times during the period of functioning of the actuator in order togenerate a high-voltage, high-frequency signal at the piezoelectricactuator from the direct-current voltage source.

Other characteristics and advantages of the invention will becomeapparent upon reading the description of several embodiments of a devicefor operating a piezoelectric actuator, illustrated by the followingfigures, wherein:

FIGS. 1 to 6 show the electronic diagram of several embodiments of aninventive operating device, according to a first structure having aDC-to-DC converter that delivers a single output voltage;

FIGS. 7 to 11 show the electronic diagram of several embodiments of aninventive operating device, according to a second structure having aDC-to-DC converter that delivers two output voltages.

For these non-limitative practical examples, elements denoted by likereference numerals on the different figures perform like functions inview of like results.

Since the invention comprises generating a sinusoidal signal having ahigh voltage greater than one hundred volts and a high frequency greaterthan ten kilohertz at the piezoelectric cell of each fuel injector of avehicle from a direct-current source, or in other words the battery, itproposes different topologies of the device for operating an actuatorfor assuring excitation of the said piezoelectric ceramics through aninductor in order to compose a resonant circuit. These structures arevalid for 1 to N injectors, where N is an integral number, preferablyequal to 4, 5, 6, 8, 10 or 12. By way of non-limitative example, thenumber of operated injectors in the description hereinafter is 4.

All the topologies described represent structures having a DC-to-DCconverter fed by the direct-current voltage source and delivering one ortwo direct-current output voltages.

According to a first structure, the DC-to-DC converter has a singleoutput between two end terminals B₁ and B₂, delivering a direct-currenthigh voltage Vs. As shown by the diagram of a first embodiment in FIG.1, the device for operating one of 4 piezoelectric actuators I_(i),where i is an integral number varying from 1 to 4, is provided with asource B of direct-current voltage E—such as a battery—between theterminals of which there is connected a DC-to-DC converter, the (−)terminal of the voltage source further being electrically grounded.Terminals B₁ and B₂ may or may not be mounted at floating potentialrelative to the battery and, in the examples described, terminal B₂ isconnected to the (−) terminal of the battery. Between the end terminalsB₁ and B₂ of converter C, which delivers a direct-current high-voltageV_(s) greater than E, there are connected a first and a second bridgearm, each composed of two alternately operable bridge switches inseries, P₁ and P₂ on the one hand and P₃ and P₄ on the other hand, andsuch that the midpoint J₁ of the first is connected to the midpoint J₂of the second by a load composed of at least one actuator in series witha resonance inductor L.

In the case of an internal combustion engine of a motor vehiclerequiring four injectors, the diagram illustrates four piezoelectricceramic actuators I₁, . . . , I_(i), . . . , I₄, which are mounted inparallel and, according to a first alternative version of the firstembodiment, are chosen successively by virtue of an operable selectionswitch K_(i) mounted in series with each of them. In the case ofinjectors having single electrical isolation, or in other words whereone of the two electrodes of the piezoelectric cell is connected to themetal frame or to a fixed potential, one of their terminals is connectedto the frame. As a function of the piezoelectric injector that must beopen during the intervals of activity to assure that the correspondingcylinder of the engine is fed with fuel, switch K_(i) is operated by alogic signal originating from the injection computer, in order that thehigh-voltage output v_(s) of the converter is connected to preciselythat injector.

In the case of injectors having piezoelectric cells with doubleisolation, or in other words where the two electrodes of each cell areisolated from the metal frame, it is also possible to invert injectorsI_(i) and selection switches K_(i) in the oscillating circuit of theactuator. As shown by the diagram of a second version of the firstembodiment in FIG. 2, where the injectors are doubly isolated, they cantherefore have one of their terminals connected directly to resonanceinductor L, and switches K₁ to K₄ have one of their terminals connectedto the frame.

The functioning of this operating circuit is as follows, as a functionof the operation of the different switches. In a first phase, theoperating signal transmitted by the injection computer on the one handcauses selection switch K_(i) connected to the chosen injector I_(i), toclose and on the other hand a first pair of bridge switches composed ofa first switch P₂ of the first arm and of a second switch P₃ of thesecond arm to close simultaneously, thus connecting midpoint J₁ of thefirst arm to terminal B₂ of converter C and midpoint J₂ of the secondarm to terminal B₁ thereof. Simultaneously, the second pair formed bythe other two switches of the said arms is operated such that theseswitches open. During this time interval, the voltage v₂ at theterminals of the resonant circuit composed of resonance inductor L andactuator I_(i) is positive, with a maximum value equal to +V_(s). Then,in a second phase, the signal operates switches P₃ and P₂ such that theyopen and simultaneously operates the two switches P₁ and P₄ such thatthey close, thus connecting midpoint J₁ of the first arm to terminal B₁of converter C and midpoint J₂ of the second arm to terminal B₂ thereof.Thus voltage v₂ at the terminals of the resonant circuit becomesnegative, with a maximum value equal to −V_(s). These two phases arerepeated a large number of times during the injection period, whichranges from 100 μs to 8 ms. Voltage v_(ci) at the terminals of injectorI_(i) is then a high-voltage, high-frequency sinusoidal signal,oscillating between a maximum value and a minimum value. The injectioncomputer then successively operates the other injectors I_(i) mounted inparallel, in the order specified for the functioning of an internalcombustion engine.

According to a second embodiment, illustrated by the diagram of FIG. 3,the four injectors I_(i) are connected in pairs by relays R₁ and R₂respectively, each connected to one terminal of selection switches K₁and K₂ respectively, the other terminal of which is connected toresonance inductor L, which is intended to compose an oscillatingcircuit with each injector in succession. The injection computer firstacts on the relays then simultaneously the selection and bridge switchesin order to select the injector to be operated.

The functioning of the operating device is as follows. For excitation ofinjector I₁, the computer first causes the means for selection of theinjector, or in other words relay R₁, to turn off relative to injectorI₁ when relay R₂ is in off position, and also causes switch K₁ to closeand switch K₂ to open for the purpose of connecting actuator I₁ toresonance inductor L. Then it simultaneously causes, in a first phase,bridge switch P₂ of the first arm to close and bridge switch P₃ of thesecond arm to open, while the other switches P₁ and P₄ are open. In asecond phase, the computer causes the four switches to change over tothe inverse position. Thus the voltage v_(c1) at the terminals ofactuator I₁ is a sinusoidal signal oscillating between the extremevalues +Vs and −Vs, while the three other injectors do not receive anyvoltage. The duration T_(Ki) of closing of each selection switchcorresponds to the injection time, which can vary between 100 μs and 5ms for a four-injector engine. The period T_(Pi) of the sinusoidalsignal v₂ at the terminals of the oscillating circuit formed by theactuator and the associated resonance inductor depends exclusively onthe structure of the injectors, the resonance frequency F_(Pi) varyingbetween 10 kHz and 1 MHz.

Since relay R₂ is already in off position, the excitation of actuator I₃is achieved by opening selection switch K₁ and closing switch K₂, sothat the voltage v₂ at the terminals of the oscillating circuit composedof inductor L and injector I₃ causes resonance thereof. The voltagesignal v_(c3) at the terminals of injector I₃ is a high-voltage,high-frequency sinusoidal signal between the following instants.

Since the switching of a relay from off position to on position takeslonger than the opening or closing of a switch, the computer causesfirst relay R₁ to switch to on position for the purpose of being able toexcite injector I₂ in the following instant. Then relay R₁ is switchedto on position while relay R₂ is still switched to off position relativeto injector I₃, and simultaneously switch K₂ is closed while switch K₁is open. And so on for the four actuators.

As in the foregoing, it is possible in an alternative embodiment toinvert the position of the actuators with that of the selection meanswhen they have double galvanic isolation, such that one of theirterminals is connected directly to the inductor.

According to a third embodiment, illustrated in the diagram of FIG. 4,wherein the DC-to-DC converter delivers a single direct-current outputvoltage V_(s), the operating device is provided with a first arm,composed of two alternately operable bridge switches P₁ and P₂ inseries, and with two alternately operable second arms, each of which iscomposed of two bridge switches P_(3j), P_(4j) in series and is mountedin parallel with the first arm between the two end terminals B₁ and B₂of converter C, where j is an integral number varying from 1 to 2.Midpoint J₁ of the first arm is connected to midpoint J_(2j) of eachsecond arm via a load composed of a group of two actuators I₁ and I₂ onthe one hand and I₃ and I₄ on the other hand, each connected to aresonance inductor L₁ and L₂ respectively.

In the precise case of FIG. 4, the load connecting the two midpoints J₁of the first arm to J_(2j) of a second arm is composed of a pair ofactuators I₁ and I₂, I₃ and I₄ in parallel, connected to relays R₁ andR₂ respectively, in turn connected to resonance inductors L₁ and L₂respectively.

In an alternative version of this embodiment, the load connecting themidpoint of the first arm to that of one of the second arms mounted inparallel is composed of two actuators mounted in parallel, such as I₁and I₂, I₃ and I₄, and each connected to an operable selection switchK_(i), where i is an integral number varying from 1 to 4, itselfconnected to a resonance inductor L_(i).

The functioning of this operating device depends on the injectioncomputer, which successively acts on the relays or the selectionswitches in such a way that the voltage obtained from voltage source Bis sufficient to achieve excitation, by an alternating square-wavevoltage, of the oscillating circuit that is formed by injector I_(i) andthe associated resonance inductor and is driven by the computer.

The fourth embodiment, illustrated in FIG. 5, relates to an inventiveoperating device, which comprises a DC-to-DC converter delivering asingle direct-current output voltage V_(s), and which is provided on theone hand with a first arm, mounted in parallel between the two endterminals B₁, B₂ of the converter and composed of two alternatelyoperable bridge switches P₁, P₂ in series, and on the other hand foursecond arms, each composed of two bridge switches P_(3i), P_(4i)respectively in series, where i is an integral number varying from 1 to4. Midpoint J₁ of the first arm is connected to midpoint J_(2i) of asecond arm via a load composed of an actuator I_(i) connected to a loadinductor L_(i).

The functioning of the device is driven by the injection computer which,before exciting an actuator I_(i), for example, operates switch P₂ ofthe first arm and switch P_(1i) of the second arm connected to actuatorI_(i), such that they close and simultaneously operates switch P₁ of thefirst arm and switch P_(4i) of the second arm such that they open in afirst phase, then in a second phase it inverts the operation of the saidswitches to deliver an exciting voltage to the oscillating circuitcomposed of actuator I_(i) and its associated resonance inductor L_(i).These two phases are repeated a specified number of times during theinjection period.

According to the fifth embodiment of the inventive operating device,illustrated by the diagram of FIG. 6, in which the DC-to-DC converteragain delivers a single direct-current output voltage V_(s), the deviceis provided with two first arms, mounted in parallel between the two endterminals B₁, B₂ of converter C and composed of two alternately operablebridge switches P₁, P₂ and P₁′, P₂′ in series, and with two second arms,composed of two bridge switches P₃, P₄ and P₃′, P₄′ respectively inseries. These arms are such that the midpoint J₁₁ and J₁₂ of each of thefirst arms is connected to the midpoint J₂₁, and J₂₂ respectively of asecond arm, via a load composed of two actuators I₁, I₂ and I₃, I₄respectively, alternately connected in parallel with a load inductor L₁and L₂ respectively. According to FIG. 6, two actuators are connected toa resonance inductor by a relay R₁ or R₂, but it is possible to connectthem by two alternately operable selection switches.

For actuators having double galvanic isolation, it is possible accordingto another version of the topology to invert the position of theactuators with that of the selection means.

To function, the injection computer operates the relay or switchcorresponding to the actuator to be excited, such as I₁, and then, in afirst phase, operates a bridge switch P₂ of the first arm connecting aterminal of the said actuator to terminal B₂ of the converter such thatit closes, and also operates bridge switch P₃ of the second armconnecting the other terminal of the said actuator to terminal B₁ of theconverter such that it closes. In a second phase, the operation of theswitches is inverted to obtain a periodic signal for excitation of theoscillating circuit composed of the actuator and its resonance inductor.These two phases are repeated a large number of times during theinjection period.

According to a second structure of the invention, the device foroperating an ultrasonic piezoelectric actuator is provided with aDC-to-DC converter that delivers two direct-current output voltagesv_(s1), v_(s2) between a common reference terminal B_(o) and two endterminals B₁, B₂, in parallel with which there is connected at least onebridge arm, composed of two alternately operable switches, the midpointof the said arm being connected to reference terminal B_(o) by a loadcomprising at least one actuator in series with a resonance inductor.The three terminals B₀, B₁, B₂ may or may not be mounted at floatingpotential relative to the battery.

FIG. 7 shows a first embodiment according to this second structure,wherein the device is provided with an arm composed of two alternatelyoperable switches P₁, P₂ in series, connected in parallel with the endoutput terminals B₁, B₂ of converter C, the midpoint J of the said armbeing connected to reference terminal B_(o) by four actuators I_(i) inparallel, where i is an integral number varying from 1 to 4, connectedalternately in series with a resonance inductor L.

According to the version of FIG. 7, each of the four actuators I_(i), afirst terminal of which is connected to midpoint J of the arm, isconnected by its other terminal to resonance inductor L via analternately operable selection switch K, the other terminal of inductorL being connected to reference terminal B_(o) of converter C. Foractuators having double galvanic isolation, it is possible to invert theposition of the actuators with that of the selection means.

According to another version, illustrated by the diagram of FIG. 8, itis possible that the four actuators I_(i), a first terminal of which isconnected to midpoint J of the arm, are connected in pairs to twooperable relays R_(j), where j varies from 1 to 2, each connected to analternately operable selection switch K_(j), which themselves areconnected to a first terminal of a resonance inductor L, the otherterminal of which is connected to reference terminal B_(o) of converterC.

The functioning of this operating circuit according to the first versionof FIG. 7 is the following, as a function of the operation of thedifferent switches. The operating signal, transmitted by the injectioncomputer in order to excite an injector I_(i), first causes thecorresponding selection switch K_(i) to close. Then, in a first phase,it simultaneously causes a first bridge switch P₂ to close and thesecond bridge switch P₁ to open, so that the voltage v₂ at the terminalsof the oscillating circuit formed by the actuator and the associatedinductor is equal to v_(s2), and in a second phase it causes the saidswitches to change over to the inverse position, in order that thevoltage v₂ is then equal to v_(s1). These two phases are then repeated alarge number of times during the injection period.

FIG. 9 shows a second embodiment according to this second structure,wherein the device is provided with two arms mounted in parallel, eachcomposed of two alternately operable bridge switches P_(1j), P_(2j) inseries, the midpoint J_(i) of the said arms being connected to referenceterminal B_(o) by a load composed of two actuators I_(i) in parallel,alternately connected in series with a resonance inductor L_(j), where jis an integral number varying from 1 to 2. In the case of this figure,each of the four actuators I_(i), a first terminal of which is connectedto the midpoint J_(j) of at least one first arm, is connected via itsother terminal to resonance inductor L_(j) via an alternately operableselection switch K_(i), the other terminal of the inductor beingconnected to reference terminal B_(o) of the converter.

According to another version of this embodiment, illustrated in FIG. 10,the four actuators I_(i), a first terminal of which is connected to themidpoint J_(i) of at least one arm, are connected in pairs to twooperable relays R_(j), each connected to a first terminal of a resonanceinductor L_(j), the other terminal of which is connected to referenceterminal B_(o) of the converter.

According to a third embodiment of this second structure, illustrated inFIG. 11, the operating device is provided with four arms, each composedof two alternately operable bridge switches P_(1i), P_(2i) in series,mounted in parallel between the two end terminals B₁, B₂, the midpointJ_(i) of which is connected to reference terminal B_(o) via a loadcomposed of an actuator I_(i) in series with a resonance inductor L_(i).

According to an essential characteristic of the invention, selectionswitches K of the actuators can be operated bidirectionallycurrent-wise, and for that purpose can be constructed from twosemiconductors mounted in series or in parallel. For example, they canbe two transistors of the MOSFET type mounted in series.

Selection relays R of the actuators are of the monostableelectromechanical type and have a break contact and a make contact.

As far as bridge switches P and P are concerned, if they are placeddirectly on the output side of the DC-to-DC converter, they are of theMOSFET, transistor or IGBT type, provided a diode is connected inanti-parallel manner in the latter case.

In all embodiments, the load inductor composing a resonant circuit withan injector is designed in such a way as to achieve the maximumresonance at the exciting frequency.

A second object of the invention is a method for use of a device foroperating at least one ultrasonic piezoelectric actuator, such asdescribed for the different foregoing topologies with the manner inwhich they function.

According to the first structure of the operating device, in which theDC-to-DC converter has a single output between two end terminals B₁ andB₂ and delivers a direct-current high voltage V_(s), the method ischaracterized in that, for operation of a given actuator, the controlcomputer on the one hand causes selection means connected to the saidactuator to close and on the other hand, in a first phase, causes afirst pair of bridge switches composed of a first switch of a first armand of a second switch of a second arm to close and simultaneously thesecond pair formed from the other two switches of the said arms to openand, in a second phase, causes the said four switches to change over toan inverse position, in such a way as to obtain a sinusoidal voltage atthe terminals of the oscillating circuit formed by the said actuator andthe associated resonance inductor, these two phases being repeated aspecified number of times during the period of functioning of theactuator in order to generate a high-voltage, high-frequency signal atthe piezoelectric actuator from the direct-current voltage source.

According to the second structure of the device for operating anultrasonic piezoelectric actuator, which is provided with a DC-to-DCconverter that delivers two direct-current output voltages v_(s1),v_(s2) between a common reference terminal B_(o) and two end terminalsB₁, B₂, the method is characterized in that, for operation of a givenactuator, the control computer on the one hand causes selection meansconnected to the said actuator to close and on the other hand, in afirst phase, causes a first bridge switch of a first arm to close andsimultaneously the second switch to open and, in a second phase, causesthe said two switches to change over to an inverse position, in such away as to obtain a sinusoidal voltage at the terminals of theoscillating circuit formed by the said actuator and the associatedresonance inductor, these two phases being repeated a specified numberof times during the period of functioning of the actuator in order togenerate a high-voltage, high-frequency signal at the piezoelectricactuator from the direct-current voltage source.

1-22. (Canceled).
 23. A device for operating at least one ultrasonicpiezoelectric actuator, which is driven electronically from a controlcomputer and a direct-current voltage source, comprising: a DC-to-DCconverter fed by the voltage source and that delivers at least onedirect-current output voltage between two end terminals; and at leasttwo bridge arms connected in parallel with the DC-to-DC converter, eachincluding two alternately operable switches in series, midpoints of thearms being connected alternately to the two end terminals of theDC-to-DC converter by a load including at least one actuator in serieswith a resonance inductor.
 24. A device for operating at least oneultrasonic piezoelectric actuator according to claim 23, wherein in acase in which the DC-to-DC converter delivers a single direct-currentoutput voltage, the device is provided with at least one first bridgearm, composed of two alternately operable bridge switches, and with atleast two second bridge arms, each including two alternately operablebridge switches and mounted in parallel with the first arm between thetwo end terminals of the DC-to-DC converter, such that a midpoint ofeach first arm is connected to a midpoint of at least one second arm bya load including at least one actuator connected to a resonanceinductor.
 25. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 24, wherein in a case in whichthe DC-to-DC converter delivers a single direct-current output voltage,the device is provided with two first bridge arms, mounted in parallelbetween the two end terminals of the DC-to-DC converter and eachincluding two alternately operable bridge switches, and with two secondbridge arms, each including two bridge switches respectively, such thata midpoint of each of the first arms is connected to a midpoint of asecond arm, by a load including two actuators in parallel, connectedalternately to a load inductor.
 26. A device for operating at least oneultrasonic piezoelectric actuator according to claim 24, wherein in acase in which the DC-to-DC converter delivers a single direct-currentoutput voltage, the device is provided with a first bridge arm, mountedin parallel between the two end terminals of the DC-to-DC converter andincluding two alternately operable bridge switches, and with two secondbridge arms, including two bridge switches respectively, such that amidpoint of the first arm is connected to a midpoint of a second arm, bya load including two actuators in parallel, connected alternately to aload inductor.
 27. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 24, wherein in a case in whichthe DC-to-DC converter delivers a single direct-current output voltage,the device is provided with a first bridge arm, mounted in parallelbetween the two end terminals of the DC-to-DC converter and includingtwo alternately operable bridge switches, and with four second bridgearms, including two bridge switches respectively, such that a midpointof the first arm is connected to a midpoint of a second arm, by a loadincluding an actuator connected to a load inductor.
 28. A device foroperating at least one ultrasonic piezoelectric actuator according toclaim 23, wherein in a case in which the DC-to-DC converter delivers asingle direct-current output voltage, the device is provided with afirst bridge arm, including two alternately operable bridge switches andconnected in parallel with output terminals of the DC-to-DC converter, amidpoint of the first arm being connected to a midpoint of a secondbridge arm, including two alternately operable switches and mounted inparallel with the first arm, by a load comprising four actuators inparallel, connected alternately to a resonance inductor.
 29. A devicefor operating at least one ultrasonic piezoelectric actuator accordingto claim 28, wherein each of the four actuators, a first terminal ofwhich is connected to a midpoint of one of the first and second armsmounted in parallel, is connected by its other terminal to a terminal ofthe resonance inductor by an alternately operable selection switch, theother terminal of the inductor being connected to a midpoint of theother arm.
 30. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 28, wherein the actuators, afirst terminal of which is connected to a midpoint of one of the firstand second arms mounted in parallel, are connected in pairs to anoperable relay, which is connected to an alternately operable selectionswitch, the switch itself being connected to a first terminal of aresonance inductor, the other terminal of which is connected to themidpoint of the other arm.
 31. A device for operating at least oneultrasonic piezoelectric actuator according to claim 25, wherein theload connects the midpoint of the first arm to the midpoint of thesecond arm, mounted in parallel, and includes two actuators in parallelconnected to a relay, the relay itself being connected to a resonanceinductor.
 32. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 25, wherein the load connectsthe midpoint of the first arm to the midpoint of the second arm, mountedin parallel, and includes two actuators in parallel and each connectedto an operable selection switch, the switch itself being connected to aresonance inductor.
 33. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 23, wherein in a case in whichthe DC-to-DC converter delivers two direct-current output voltages, thedevice is provided with two bridge arms in parallel, each including twoalternately operable bridge switches, a midpoint of the arms beingconnected to a reference terminal by a load including two actuators inparallel, alternately connected in series with a resonance inductor. 34.A device for operating at least one ultrasonic piezoelectric actuatoraccording to claim 33, wherein each of the four actuators, a firstterminal of which is connected to the midpoint of at least one bridgearm, is connected by its other terminal to the resonance inductor by analternately operable selection switch, the other terminal of theinductor being connected to a reference terminal of the converter.
 35. Adevice for operating at least one ultrasonic piezoelectric actuatoraccording to claim 33, wherein the four actuators, a first terminal ofwhich is connected to the midpoint of at least one bridge arm, areconnected in pairs to two operable relays, each connected to a firstterminal of a resonance inductor, the other terminal of which isconnected to a reference terminal of the DC-to-DC converter.
 36. Adevice for operating at least one ultrasonic piezoelectric actuatoraccording to claim 33, wherein the four actuators, a first terminal ofwhich is connected to the midpoint of at least one bridge arm, areconnected in pairs to two operable relays, each connected to analternately operable selection switch, themselves connected to a firstterminal of a resonance inductor, the other terminal of which isconnected to a reference terminal of the converter.
 37. A device foroperating at least one ultrasonic piezoelectric actuator according toclaim 33, wherein in a case in which the DC-to-DC converter delivers twodirect-current output voltages the device is provided with four bridgearms, each including two alternately operable bridge switches mounted inparallel between the two end terminals, a midpoint of which is connectedto a reference terminal by a load including an actuator in series with aresonance inductor.
 38. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 23, wherein the voltage sourceis a low-voltage battery or an alternator.
 39. A device for operating atleast one ultrasonic piezoelectric actuator according to claim 23,wherein the selection switches of the actuators are configured to beoperated bidirectionally current-wise, and are constructed from twosemiconductors mounted in series or in parallel.
 40. A device foroperating at least one ultrasonic piezoelectric actuator according toclaim 23, wherein the selection relays of the actuators are of themonostable electromechanical type and have a break contact and a makecontact.
 41. A device for operating at least one ultrasonicpiezoelectric actuator according to claim 23, wherein the bridgeswitches are placed directly on an output side of the DC-to-DCconverter, and are of MOSFET or IGBT or transistor type.
 42. A methodfor use of a device for operating at least one ultrasonic piezoelectricactuator according to claim 23, wherein for operation of a givenactuator, a control computer causes a selector connected to the actuatorto close and, in a first phase, causes a first pair of bridge switchesincluding a first switch of a first arm and of a second switch of asecond arm to close and simultaneously the second pair formed from theother two switches of the arms to open and, in a second phase, causesthe four switches to change over to an inverse position, to obtain asinusoidal voltage at terminals of an oscillating circuit formed by theactuator and an associated resonance inductor, the two phases beingrepeated a specified number of times during a period of functioning ofthe actuator to generate a high-voltage, high-frequency signal at theactuator from the direct-current voltage source.
 43. A method for use ofa device for operating at least one ultrasonic piezoelectric actuatoraccording to claim 23, for operation of a given actuator, a controlcomputer causes a selector connected to the actuator to close and, in afirst phase, causes a first bridge switch of a first arm to close andsimultaneously the second switch to open and, in a second phase, causesthe two switches to change over to an inverse position, to obtain asinusoidal voltage at terminals of an oscillating circuit formed by theactuator and an associated resonance inductor, the two phases beingrepeated a specified number of times during a period of functioning ofthe actuator to generate a high-voltage, high-frequency signal at theactuator from the direct-current voltage source.